1. Technical Field
This invention relates in general to semiconductor devices and, more particularly, to mounting TAB (Tape Automated Bonding) packaged microchips on a circuit board.
2. Description of the Prior Art
Modern integrated circuits typically use multiple transistors fabricated in a single crystal silicon substrate containing multiple levels of metallization for interconnections. In addition, modern microchips have numerous electrical elements that make up part of the integrated circuit. As electrical current runs through these elements, heat is generated and, consequently, efficient dissipation of the heat becomes a major concern. Thermal management is thus a major design requirement for modern microchips.
Increased miniaturization in the electronic industry has resulted in more and more components being placed on circuit boards in the form of TAB (Tape Automated Bonding) packages. The most general method of attaching the leads of the TAB packages to a printed wiring board (PWB) is to use the "formed lead" process. In this process, the component leads are first excised and formed into a gull-wing shape. Then the leads are attached to the pads on the board using a hot bar or thermocompression bonding method. The leads on the TAB device are formed in such a way to accommodate the thickness of the die attach layer (i.e., material used for contact between the die and the board for heat dissipation and electrical contact) placed between the die and the PWB. Accordingly, the process requires a consistent thickness of the die attach layer for the leads to be formed correctly. Additionally, warpage of the board may result in non-uniform thickness of the die attach layer, with unfavorable consequences.
Another attachment process of a TAB package to a PWB is the "no form process", where the leads of the package are excised, but not formed. In this process, leads act much like cantilever beams which are bent down using a hot bar. The thickness of the die attach layer is less critical in this method since the cantilever-shaped leads are bent down during the bonding process and allows for larger tolerance in die attach layer thickness.
One use for the die attach material is to provide an electrical connection between the die and the PWB, typically for grounding purposes. Electrically conductive die attach materials are usually silver filled polymers; they also function as the thermal path for heat dissipation. Since TAB devices are not encapsulated at the backside of the components, and thus have the leads exposed, any migration of the silver, or other conductive material, to the inner leads during attachment will result in shorting between leads. To avoid migration of the die attach along the sides of the die, the conductive die attach layer is cut to an area smaller than the die itself, to restrict the material from migrating to the edge of the die. A smaller area of die attach can have severe consequences, as it reduces heat flow and, consequently, the rate of heat dissipation. As a result, the device may overheat, causing reduced performance or device failure.
Different approaches have been attempted to overcome this problem by using backside encapsulation of the TAB leads. One approach for backside encapsulation of the TAB leads uses materials similar to a solder mask for the encapsulation. This approach has many problems. First, the flow of solder mask is difficult to control and accurate dispensing of the material has been found to be not repeatable. Another problem involves the high CTE (coefficient of thermal expansion) and elastic modulus mismatch of the solder mask material and the polyimide film used in the TAB component to hold the leads coming out of the TAB device. The polyimide film warps significantly after the solder mask cures, mainly due to the bimetallic strip effect of materials with different expansion rates. Due to excessive warpage of the polyimide film after encapsulant cure, the automated component placement machines can not inspect or find the leads during the placement process (since the component warpage is more than the depth of focus of the camera), making the components non-manufacturable.
Another approach uses material similar to the die encapsulation material for backside encapsulation of the die leads. This approach shows the same polyimide warpage as in the case of solder mask material discussed above. Accordingly, it inhibits accurate lead placement by the automated component placement machines.
Therefore, there is a need in the industry for a method of preventing electrical shorting of die leads during installation of a TAB device on a circuit board.